The present invention relates generally to spectroscopic imaging systems and, more particularly, to a method and apparatus for enhancing the overall efficiency of Raman, fluorescence, photoluminescence, ultraviolet, visible, and infrared absorption spectroscopic imaging systems employing polarization sensitive imaging spectrometers.
Spectroscopic imaging combines digital imaging and molecular spectroscopy techniques, which can include, Raman scattering, fluorescence, photoluminescence, ultraviolet, visible and infrared absorption spectroscopies. When applied to the chemical analysis of materials, spectroscopic imaging is commonly referred to as chemical imaging. Instruments for performing spectroscopic (i.e. chemical) imaging typically comprise image gathering optics, focal plane array imaging detectors and imaging spectrometers.
In general, the sample size determines the choice of image gathering optic. For example, a microscope is typically employed for the analysis of submicron to millimeter spatial dimension samples. For larger objects, in the range of millimeter to meter dimensions, macro lens optics are appropriate. For samples located within relatively inaccessible environments, flexible fiberscopes or rigid borescopes can be employed. For very large scale objects, such as planetary objects, telescopes are appropriate image gathering optics.
For detection of images formed by these various optical systems, two-dimensional, imaging focal plane array (FPA) detectors are typically employed. The choice of FPA detector is governed by the spectroscopic technique employed to characterize the sample of interest. For example, silicon (Si) charge coupled device (CCD) detectors, a type of FPA, are typically employed with visible wavelength fluorescence and Raman spectroscopic imaging systems, while indium gallium arsenide (InGaAs) FPA detectors are typically employed with near-infrared spectroscopic imaging systems.
A variety of imaging spectrometers have been devised for spectroscopic imaging systems. Examples include, without limitation, grating spectrometers, filter wheels, Sagnac interferometers, Michelson interferometers and tunable filters such as acousto-optic tunable filters (AOTFs) and liquid crystal tunable filters (LCTFs).
A number of imaging spectrometers, including AOTFs and LCTFs are polarization sensitive, passing one linear polarization and rejecting the orthogonal linear polarization. As a result, theoretical efficiency is 50%. Practical efficiency is reduced due to scattering losses, imperfect spectrometer action, absorption losses in polarizing optics, etc. Practical efficiency of 30% peak transmittance or less is more typical. Previous spectroscopic imaging instruments accepted the optical losses associated with polarization sensitive imaging spectrometers.
In a paper by Patrick Treado, et. Al. entitled xe2x80x9cHigh-Fidelity Raman Imaging Spectrometry: A Rapid Method Using an Acousto-Optic Tunable Filterxe2x80x9d Applied Spectroscopy (1992)-46, 1211, the authors describe the use of an AOTF imaging spectrometer for Raman spectroscopic imaging. The instrumental approach uses a polarization sensitive imaging spectrometer to provide spectral tuning of Raman images. In operation, over 60% of the Raman scattered radiation is rejected by the optical system, with the majority of the loss being due to polarized Raman scattered radiation that is not captured through the polarization sensitive tunable filter.
A variety of spectroscopic imaging systems have been devised that make use of the polarized spectroscopic information typically rejected by polarization sensitive imaging spectrometer. For example, in a paper by Hannah Morris, et. al. entitled xe2x80x9cLiquid Crystal Tunable Filter Raman Chemical Imaging,xe2x80x9d Applied Spectrosc. (1996)xe2x80x94Morris, Hannah R.; Hoyt, Clifford C.; Miller, Peter; and Treado, Patrick J. xe2x80x9cLiquid Crystal Tunable Filter Raman Chemical Imaging,xe2x80x9d Appl. Spectrosc. 50 (1996) 805-811, the authors describe a polarization sensitive imaging spectrometer that operates to collect Raman spectroscopic image information. The imaging spectrometer is generally efficient at gathering wavelength-resolved spectroscopic imaging information, but less efficient at rapidly gathering full spectral information. However, for rapid acquisition of non-imaging Raman spectral data the output of the microscope is coupled either via direct beam or fiber optic to a dispersive Raman spectrograph. In order to select between imaging and non-imaging spectral data collection modes, movable mirror assemblies are typically used. Both operating modes are generally useful in spectroscopic imaging systems, whereas the imaging spectrometer mode is optimized for image characterization of samples of interest, while the non-imaging spectral mode is effective for spectral characterization of samples of interest.
In U.S. Pat. No. 6,002,476, a polarization sensitive imaging spectrometer having enhanced transmission efficiency is disclosed. Despite the enhanced transmission efficiency of the imaging spectrometer, over 50% of the light is lost to the polarization order not detected by the imaging detector.
In U.S. Pat. No. 5,689,333, one means to improve the overall efficiency of a Raman microscope that can be employed for spectroscopic imaging using filter wheels, is to make effective use of holographic laser rejection filters as beamsplitters for efficient coupling of the laser source into the microscope. To those practiced in the art of Raman microscopy, the use of holographic filters is thought to be an essential component of high performance Raman instrumentation.
To address the need for improved overall performance efficiency for spectroscopic imaging systems reliant upon polarization sensitive imaging spectrometers that discard up to 50% of the light absorbed, emitted or scattered by the sample, a novel apparatus has been developed that has been incorporated within spectroscopic imaging instruments. This apparatus employs a polarization beamsplitter to couple the light typically discarded by the polarization sensitive imaging spectrometer to a non-imaging spectrometer to simultaneously record a spectrum of the sample. The design of the simultaneous imaging and spectroscopy apparatus has several distinct advantages over the prior art:
First, the polarization beamsplitter allows simultaneous spectroscopic imaging and spectroscopy to be performed without the need for moving mechanical parts or time sequenced sampling. This advantage is particularly useful in rapid sample screening applications where minimizing sampling time is beneficial.
Second, the polarization beamsplitter is incorporated within the infinity-corrected optical pathlength of the spectroscopic imaging system, without introducing significant optical signal loss. The added capability of simultaneous spectral sampling is provided without degrading the spectroscopic imaging capability, effectively doubling the overall optical efficiency of the spectroscopic imaging system integrated with non-imaging spectrometer technology.
Third, the polarization beamsplitter affords a more compact design than movable mirror designs, as well as improved overall ruggedness of optical design which is important for use in high vibration environments.
Fourth, the dielectric filters employed as beamsplitters to couple laser excitation into microscope systems provides an improved angular field of view relative to more conventional holographic filter implementations. As a result, for wide field of view laser source illumination, improved laser background rejection efficiency is provided, as well as reduced sensitivity to angular alignment. Reduced sensitivity to angular alignment is beneficial since it improves the ruggedness of the optical system and the overall ease of alignment. This in turn reduces the cost of manufacturing and instrument maintenance.
The various aspects of the present invention may be more clearly understood and appreciated from a review of the following detailed description of the disclosed embodiments and by reference to the appended drawings and claims.